Many electronic weighing scales use electrical sensors to provide electrical signals indicative of a weight applied to the scale. For example, see co-pending U.S. patent application, Ser. No. 08/385,349, entitled "ELECTRICAL WEIGHING SCALE", by Petrucelli et al., filed on Feb. 8, 1995, and assigned to Measurement Specialties, Inc.
Most of these electronic scales also use a set of levers which transmit an applied load to a single load cell. The load cell is typically constructed as a mechanically-deformable element which operates as a force transducer and an electronic strain sensor. When a load is applied to such a load cell, the load cell mechanically deforms and produces an electrical signal which is proportional to the load applied to the load cell. These lever arrangements, however, are overly complex, require close tolerance components and must be properly aligned to function accurately.
Attempts have been made in the prior art to construct scales without levers. One such design includes a scale with a plurality of load cells. When a load is applied to the scale, the load is distributed among all the load cells. Electrical signals generated by each of the load cells are then summed up to obtain an accurate measure of the total load on the scale. Several patents have been issued for electronic scales which embody this principle. For example, see U.S. Pat. No. 4,040,686 issued to Brendel and U.S. Pat. No. 4,411,327 issued to Lockery.
Brendel and Lockery both use a well-known double-cantilever arrangement for retaining precision under eccentric load conditions. In particular, two strain gauges are bonded to a flexure beam, whereby upon the application of a load, one gauge is placed in tension and the other gauge is placed in compression of an equal magnitude, so that additional moments created by transverse forces are cancelled. The signal generated by the two strain sensors bonded to the flexure beam is proportional to the sum of the bending moments on the beam at the center points of the gauges. Since the sensors are located on the beam at locations which are equi-distant from the beam's mid-point, a force pressing on the leading edges of the beam will be proportional to the product of the force and the distance between the sensors. Since the distance between the sensors is fixed, the signal will be proportional to the force even if the force is not exactly at the center of the flexure beam.
The load cells described in Lockery and Brendel, however, are not easily mass produced. They each contain a relatively large number of parts and each require considerable machining, grinding, tapping, screwing, assembly and post-mounting trimming operations which substantially increase their manufacturing costs.
Another type of load cell is disclosed in U.S. Pat. No. 4,993,506 issued to Angel. This patent discloses a load cell which uses a flat flexure beam on which strain sensors are bonded. A flat U-shaped loading element is attached to one end of the flexure beam and a flat mounting element is attached to the other end of the flexure beam. The problem with this type of load cell is that it also requires several mounting operations.
Accordingly, it is an object of the present invention to provide an improved load cell for a scale that is highly reliable yet substantially simple and economical to manufacture.